Now the wavelength-conversion lineup is ready for long-distance quantum communications.

Here, we present our latest research published in Nature Communications (DOI number:10.1038/ncomms1544) in November 2011. Recently, quantum information processing is widely investigated such as quantum communication and quantum computation. Imoto laboraroty aims at developing new quantum information processing both from theoretical and experimental sides. Especially, we lay stress on the interplay between photonic quantum signals and matter-based quantum memories. Here we describe our recent result on the photonic frequency down conversion keeping the quantum information of the photon with a high fidelity. Quantum information transfer via the optical frequency conversion is very important in the future practical quantum communications since there is the wavelength mismatch between the optical signals and the quantum memories: 1.3-1.5 micron wavelength is used in the optical fiber transmission while visible to very near infrared light is used for the matter memories. So far wide-band (that is, high-speed) frequency up-conversion and narrow-band down-conversion keeping quantum information have been demonstrated, while wide-band frequency down-conversion keeping quantum information has not been demonstrated. In our work, we experimentally demonstrated wide-band frequency down conversion keeping the quantum information of the photon with a high fidelity. We used a similar technology as some of the preceding studies, that is, use of a periodically poled LiNbO3 (called “PPLN”). We used, however, a high quality PPLN together with our photonic-quantum-information technology accumulated through our experiments published in two nature series and many Phys. Rev. series publications. This brought our experiment to the success. Our research result opens a new root to long-distance quantum communications and high-speed quantum information processing.